Fiber optics offers the avionics industry advantages of weight and stealth compared to traditional wiring. Local area fiber cables can provide a means of communication between the various avionics modules. The Air Force recognizes those advantages and has included fiber optics in the design of the F/A-22 aircraft and will continue to design fiber optics into aircraft upgrades and new aircraft designs. The ability to repair broken fibers will be critical to keeping those systems flight worthy.
As with standard wiring, it is very difficult to pull and replace complete runs of fiber cable within an aircraft. A much more attractive solution is to repair the cable at the damaged location. Standard fiber optic splice technology, however, is not adequate for avionics applications. The temperature range, mechanical forces, humidity variations and environmental conditions including salt, sand and smoke encountered in fighter jet operating conditions, cause standard fiber optic splices, such as those used in the standard telecommunications field, to fail.
There is therefore a need for an avionic fiber optic splice and splicing method for use in Aircraft Battle Damage Repair (ABDR), peace time repair, and maintenance of aging aircraft. The splice must be easy to install, must be installable in tight working conditions, must have a small cross section to avoid the necessity of staggered repairs, and must perform under the conditions experienced during fighter jet flights. A fiber splice that meets those requirements will find applications in all branches of military aircraft as well as within commercial aircraft.
The methods, splices and tools of the present invention were developed for use in ABDR on the F/A-22 aircraft, and are suitable for use with other aircraft and maintenance programs. They were developed to meet the following requirements.
1) The splice must be easy to install in the severely limited space encountered within the fighter jet.
2) The splicing method must provide visual feedback during the repair process in order to assure proper assembly and performance of the splice.
3) The cross section of the splice must be minimized in order to minimize the need of staggered splices. The use of staggered splices requires two splices per repair and each splice causes a loss in signal of 30% on average.
4) The splice must satisfy military performance specifications as set forth in MIL-PRF-24623C.
The performance requirements for an ABDR repair are less stringent than those for peace time repair. Basically, the performance of a peace time repair must match the original performance specifications. The ABDR repair, on the other hand, must quickly get the aircraft flight worthy but may have lower performance and lifetime specifications.
Fiber optic splices may be classified into four types: 1) multimode mechanical splice, 2) multimode fusion splice, 3) single mode mechanical splice, and 4) single mode fusion splice. “Single” and “multimode” are types of optical fiber. “Mechanical” and “fusion” are methods of splicing. The goal of the inventors in developing the present invention was to develop a multimode mechanical splice. Features of the invention, however, are suitable for use in any of the fiber optic splice classes.
A problem often encountered in the use of presently available fiber optic splicing techniques for ABDR is that the fibers to be spliced must be cleaved to a very precise length tolerance, and that the fiber ends must be perpendicular and smooth in preparation for splicing. In presently used techniques, the fiber ends are polished to meet those requirements. Those procedures present significant difficulties in an ABDR environment where working space is limited and time is of the essence.
Another difficulty with fiber optic splicing techniques presently available for avionics is that there is no real-time feedback to the operator regarding the actual alignment and positioning of the two cleaved fiber ends with respect to each other. That results in inconsistent results in splice quality, as measured by transmission loss through the splice. Such losses are not measurable until the splice is completed, and the entire splicing process must be repeated if the results are substandard.
Presently available techniques produce splices having outer diameters exceeding 0.20 inches, requiring the staggering of splices in many instances. Presently available techniques furthermore require the use of expensive custom-made tools, raising the per-splice overall cost.
In mechanical fiber optic splices, the glass fibers ends must be held securely in place against each other in the splice. That is currently done using high-strength adhesives to secure the glass fibers in sleeves that are mounted in the crimp assembly. Adhesives, however, are difficult to use under tight working conditions and have limited shelf life, requiring careful inventory control.
Frequently, fiber optic splice specifications, especially those used in peace-time applications, require that the splice meet minimum pull test standards. Meeting such standards almost always requires that the splice connects with a strength member of the cable in or immediately beneath the outer jacket. Known arrangements for connecting to the strength member, however, require a splice outer diameter that greatly exceeds the diameter of the fiber cable itself. If several adjacent fiber cables require splicing, those splices must then be staggered to avoid an excessively large overall diameter of the splice group. That situation may necessitate multiple splices on some cables.
There therefore remains a need for a cost-effective, reliable technique to splice optical fiber cables in an ABDR environment.